As energy prices are increasing due to the depletion of fossil fuels and increasing attention is being paid to environmental pollution, the demand for environmentally friendly alternative energy sources is bound to play an increasing role in the future. Thus, research into techniques for generating various kinds of power, such as nuclear energy, solar energy, wind energy, and tidal power, is underway, and power storage apparatuses for more efficient use of the generated energy are also drawing much attention.
In particular, the demand for secondary batteries as energy sources is rapidly increasing as mobile device technology continues to develop and the demand for such mobile devices continues to increase. Accordingly, much research on batteries satisfying various needs has been carried out.
In general, a secondary battery is a battery that can be charged and discharged, unlike a primary battery, which is not chargeable. The secondary battery is widely used in electronic devices, such as mobile phones, camcorders, and laptop computers, or electric vehicles. In particular, a lithium secondary battery has a capacity three or more times larger than the capacity of a nickel cadmium battery or a nickel hydride battery, which is widely used as a power source for electronic devices, and exhibits high energy density per unit weight. For these reasons, the lithium secondary battery has been increasingly used.
Based on the shape of the battery case of a secondary battery, the secondary battery may be classified as a cylindrical battery configured to have a structure in which an electrode assembly is mounted in a cylindrical metal battery can, a prismatic battery configured to have a structure in which an electrode assembly is mounted in a prismatic metal battery can, or a pouch-shaped battery configured to have a structure in which an electrode assembly is mounted in a pouch-shaped battery case made of an aluminum laminate sheet.
A cylindrical secondary battery generally includes a cylindrical battery can, an electrode assembly mounted in the cylindrical battery can, and a cap assembly coupled to the upper part of the cylindrical battery can. The cap assembly is located at an opening formed in the upper part of the cylindrical battery can. The cap assembly includes a top cap and a safety vent.
FIG. 1 is a vertical sectional view showing a cap assembly of a conventional cylindrical secondary battery.
Referring to FIG. 1, the conventional cylindrical secondary battery includes a cylindrical battery can 20, a jelly-roll type electrode assembly 30 mounted in the battery can 20, a cap assembly 10 coupled to the upper part of the battery can 20, a beading part 40 provided at the upper end of the battery can 20 for allowing the cap assembly 10 to be loaded on the upper part of the battery can 20, and a crimping part 50 for sealing the battery.
The electrode assembly 30 is configured to have a structure in which a positive electrode and a negative electrode are wound in the form of a jelly roll in the state in which a separator is interposed between the positive electrode and the negative electrode. A positive electrode lead 31, which is attached to the positive electrode, is connected to the cap assembly 10, and a negative electrode lead (not shown), which is attached to the negative electrode, is connected to the lower end of the battery can 20.
The cap assembly 10 is configured to have a structure in which a top cap 11, which constitutes a positive electrode terminal, a safety element 12 for interrupting the flow of current by increasing battery resistance when the temperature in the battery increases, such as a positive temperature coefficient (PTC) element, a safety vent 13 for interrupting the flow of current and/or discharging gas to the outside when the pressure in the battery increases, a current interrupt device (CID) gasket 14 for electrically separating the safety vent 13 from a current interrupt device 15 excluding a specific region of the secondary battery, and the current interrupt device 15 connected to the positive electrode lead 31, which is attached to the positive electrode, are sequentially stacked. The cap assembly 10 having the above-described structure is mounted to the beading part 40 of the battery can 20 in the state in which a sealing gasket 16 is interposed therebetween.
Under normal operating conditions, therefore, the positive electrode of the electrode assembly 30 is connected to the top cap 11 via the positive electrode lead 31, the current interrupt device 15, the safety vent 13, and the safety element 12, whereby the secondary battery is in an electrically conductive state.
When gas is generated in the battery can 20 due to overcharge, etc. and thus the pressure in the battery can 20 increases, however, the safety vent 13 protrudes upward while being deformed. As a result, the safety vent 13 is separated from the current interrupt device 15, whereby the flow of current is interrupted.
Consequently, the charge and discharge of the secondary battery is no longer performed, whereby the safety of the secondary battery is secured. Furthermore, when the pressure in the secondary battery exceeds a predetermined level, the safety vent 13 ruptures, and the gas in the battery can is discharged to the outside through a gas hole (not shown) formed in the top cap 11 via the ruptured safety vent, whereby the explosion of the secondary battery is prevented.
In the above structure, the current interrupt device 15 is generally made of a metal material. When an external strong impact or pressure is applied to the secondary battery or when the pressure of the gas in the secondary battery increases, the CID gasket 14 is separated or detached from the current interrupt device 15 or the CID gasket ruptures. As a result, the current interrupt device 15 comes into contact with the battery can or the negative electrode, whereby a short circuit may occur in the secondary battery. In addition, when the current interrupt device 15 comes into contact with the safety vent 13, the current interrupt device 15 cannot interrupt the flow of current, whereby the secondary battery may explode, which drastically reduces the safety of the secondary battery.
Conventionally, the CID gasket 14 is made of an insulative material, such as polypropylene (PP) or polybutylene terephthalate (PBT) in order to prevent the occurrence of a short circuit in the secondary battery. When the secondary battery is deformed by external pressure, however, the CID gasket 14 is 14 is separated or detached from the current interrupt device 15 or the CID gasket ruptures, making it impossible to prevent the occurrence of a short circuit in the secondary battery.
Therefore, there is a strong necessity for technology that is capable of preventing a short circuit from occurring due to the current interrupt device 15, thereby improving the safety of the secondary battery.